MUSEUM OF COMPARATIVE ZOOLOGY. 249 



body, and contain very large yolk spherules. The majority of the ento- 

 dermal cells contain no conspicuous accumulations of pigment ; but the 

 latter may occasionally be found in considerable quantity, particularly in 

 the cells bordering on the gut. 



In the dorsal portion of the body, the mesoderm consists of two lateral 

 masses of tissue, each of which spreads outward and ventraJward from 

 the neural tube, and joins its fellow of the opposite side in the ventral 

 median line. Each of these masses of mesoderm is thickest next to 

 the medullary tube, and gradually becomes thinner in passing outward 

 around the mass of yolk cells. In the dorsal half of the body (Fig. 48) 

 each mass of cells consists of two distinct layers, which are continuous 

 with each other along the sides of the neural tube. They represent the 

 first division into somatic (la. so.) and splanchnic (la. spl.) mesoderm, and 

 the slight space which separates them is the coelom (cce/.). On passing 

 outward and ventrally, the two layers of mesoderm gradually approach, 

 and at length are continuous with, each other ; for a short distance 

 farther, it is still possible to trace two rows of nuclei, indicating approx- 

 imately the territory occupied by the layers ; but this arrangement 

 finally disappears, and before the ventral surface is reached the meso- 

 derm has the form of a layer only one cell in thickness (ms'drm.). 



In both somatic and splanchnic layers, the cells are of a nearly cubical 

 form, but those of the parietal layer are rather thicker, and may be even 

 columnar. The mesoderm of the ventral side of the body, on the other 

 hand, is composed of more flattened elements. The cells of the meso- 

 derm are in general intermediate in size between those of the ectoderm 

 and of the entoderm. Their yolk spherules are much smaller than those 

 in the entoderm, but resemble those in the ectoderm too closely to af- 

 ford a thoroughly satisfactory criterion for distinguishing the two layers. 

 The mesodermal yolk spherules are, however, slightly larger than those of 

 the ectoderm ; and in doubtful cases they may be taken into account. 



The pigment of the mesoderm is usually collected along that surface 

 of the cell which faces the coelom, and may in part serve as a guide for 

 following that cavity in cases where the bounding layers of mesoderm are 

 in close contact with each other. 



I have spoken of the somatic mesoderm as a layer a single cell in 

 thickness; this is not, however, an adequate representation of the actual 

 condition. In many sections there may be observed, from place to place, 

 an additional cell associated with the otherwise single layer. The occur- 

 rence of an incomplete second layer of cells is most noticeable in the 

 anterior portion of the trunk, in a region directly lateral to the protover- 



